As far as I am aware, they appear from the soil and lay eggs in the centre of the bush. when the eggs hatch 3 weeks later they eat the leaves. There are often more than one lifecycles a year.
This being the case, it's not really a matter of netting the bush, but preventing the developing pupae from getting into the soil and then migrating up the plant.
So...a cover over the soil under the bush would be the way forwards ( bit like a huge cabbage cuff). Maybe netting might stop the odd one migrating- but I'm not quite so sure if that's the usual thing which happens???

This is what I've understood to be the case- but never really had a problem, so I've never had to try out the theory!